The subject of the present invention is a measuring unit for determining the countervoltage or bucking voltage of electric motors which is particularly well suited for controlling d-c machines which are supplied with an armature current obtained by phase gating from a thyristor-controlled three-phase rectifier bridge.
Direct current motors which are employed in machine tools are subject, on the one hand, to heavy load variations and must be adapted, on the other hand, to rapidly varying operating conditions. This requires a control with the aid of a parameter which allows the determination of the prevailing operating state of the motor in a particularly characteristic manner According to experience, the bucking voltage generated in the motor is suited best as the parameter, which depends on the magnetic excitation as well as on the speed of the motor.
As to the present state of the art, it can be stated that the control of d-c machines by controlled rectifier bridges is known through U.S. Pat. No. 3,593,087, where, however, the armature voltage is measured and not calculated. This applies also to the control for d-c machines described in U.S. Pat. Nos. 3,683,252, 4,356,434 on the controlled by a processor, no determination of the bucking voltage is involved.
For an explanation of the relationships required for an understanding of the invention, the basic circuit diagram of the speed control device of a d-c machine supplied from a three phase network by means of a thyristor controlled rectifier bridge is shown in FIG. 1 of the drawing which substantially also corresponds to the circuits which are shown in IEEE Transactions on Industry Applications Vol. IA-16, No. 3 (June 1980), "A Performance Analysis of Microprocessor-Based Control
Systems . . . ", pages 378 to 387 in FIGS. 1 and 3. However, FIG. 1 of the drawing also shows, in addition to the circuit known from the prior art, the measuring unit according to the invention for determining the bucking voltage as well as a pilot control device serving to achieve a high dynamic range of 12 the current control circuit, which makes the knowledge of the bucking voltage necessary. The circuit shown is realized in digital form in a microprocessor, where the control lines shown are understood to be information flow lines.
The machine speed to be preset in each case is applied as the reference value n.sub.s to the input of the circuit and is compared in a difference member D1 with the actual speed value n.sub.i. The speed control deviation obtained as the difference of both values is fed in a first speed control loop to a first controller R1 which furnishes from its output a current reference value I.sub.s corresponding to the speed control deviation. By means of a second difference member D2, this current reference value I.sub.s is compared with an actual current value I.sub.i and the current control deviation resulting as the difference of both values is fed in a second subordinated current control loop to a second controller R2 which furnishes at its output a value .alpha..sub.1 corresponding to the current control deviation for the phase gating angle .alpha.. The controller R2 is usually a PI controller, the output variable of which is utilized for controlling the rectifier bridge GB which feeds the directly connected d-c machine GM. Into the armature circuit of the d-c machine GM is inserted a d-c transformer GW which furnishes the prevailing actual current value I.sub.i to the difference member D2 in binary coded form. The prevailing actual speed value n.sub.i of the d-c machine GM measured by a tachometer generator TD is furnished to the difference member D1 also in binary coded form.
A speed change of the d-c machine GM, using the circuit elements described so far, of the speed control device, follows a fast changing speed reference value n.sub.s relatively slowly. This can be improved by making the phase gating angle dependent not only on the phase current control deviation (I.sub.s -I.sub.i), but additionally also on the bucking voltage E induced in the armature of the d-c machine GM and the current reference value I.sub.s. To this end, the relatively high armature voltage could be taken off directly at the terminals of the d-c machine, then be separated d-c wise from the machine, subjected after smoothing to an analog to-digital conversion and be corrected by the ohmic armature voltage drop. This form of determining the bucking voltage would require additional hardware and be heavily delayed by the necessary smoothing and would therefore not be suited for a highly dynamic control.